Perpendicular recording can support much higher linear density than longitudinal recording due to lower demagnetizing fields in recorded bits, which diminish as linear density increases. To provide decent writeability, double layer media are used. The double layer perpendicular media consist of a high coercivity, thin storage layer with perpendicular to-plane anisotropy and a soft magnetic keeper (underlayer) having in-plane anisotropy and relatively high permeability.
A magnetic head for perpendicular recording generally consists of two portions, a writer portion for writing magnetically-encoded information on a magnetic media (disc) and a reader portion for retrieving magnetically-encoded information from the media. The reader portion typically consists of a bottom shield, a top shield, and a sensor, often composed of a magnetoresistive (MR) material, positioned between the bottom and top shields. Magnetic flux from the surface of the disc (media) causes rotation of the magnetization vector of a sensing layer of the MR sensor, which in turn causes a change in electrical resistance of the MR sensor. The change in resistance of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover the data that was encoded on the disc.
The writer portion of the magnetic head for perpendicular recording typically consists of a main pole and a return pole which are magnetically separated from each other at an air bearing surface (ABS) of the writer by a non-magnetic gap layer, and which are magnetically connected to each other in a region at a distance away from the ABS. Positioned at least partially between the main and return poles are one or more layers of conductive coils encapsulated by insulating layers. The ABS is the surface of the magnetic head immediately adjacent to the perpendicular medium. The writer portion and the reader portion are often arranged in a merged configuration in which a shared pole serves as both the top or bottom shield of the reader portion and the return pole of the writer portion.
To write data to the magnetic media, an electrical current is caused to flow through the conductive coil, thereby inducing a magnetic field across the write gap between the main and return poles. The main and return poles are made of soft magnetic materials. Both the main and return pole may generate magnetic field in the media during recording when the write current is applied to the coil. However, the main pole produces a much stronger write field than the return pole by having a much smaller cross-sectional area at the ABS. A magnetic moment of the main pole should be oriented along an easy axis parallel to the ABS when the main pole is in a quiescent state, namely without a current field from the write coil. When the magnetic moment does not return to an orientation parallel to the ABS after being subjected to one or multiple excitations of the write current field, the main pole is not stable. In an unstable pole, the orientation of the magnetic moment generally remains nonparallel to the ABS even after current to the write coil is turned off. Thus, the main pole in the quiescent state may still emit a magnetic flux and may deteriorate or even erase data from the disc. Further, an unstable pole results in increased switching time when a write current is applied. In a perpendicular head, the main pole is a predominant source of remanent magnetism due to a strong shape anisotropy perpendicular to the ABS.
Accordingly, there is a need for a writer with minimal remanent magnetization when the write current is switched off. Such a stable writer will reduce switching time, increase data rate of the disc drive, and prevent unintentional erasing on perpendicular media after the write current is turned off.